Access systems for large earth moving equipment often incorporate a set of deployable stairs so that the operator can climb up into the cab without climbing a vertical ladder that may have a negative inclination if the equipment is not parked on level ground.
Once the operator is on the machine the operator retracts the stairs so that they are not hanging down and the ground clearance of the machine is increased thus minimizing the probability of the stairs being damaged by contacting rocks when the machine is in motion.
The stairs need to be retracted so that other personnel cannot climb up onto the machine without the operator's knowledge. Should a person be on the stairs without the operator's knowledge then the possibilities for injury to that person are numerous. The possible injuries could range from overbalancing the person to crushing or trapping the person between the moving parts of the equipment and the adjoining bodywork of the equipment.
The systems used for retracting the stairs can vary but generally they rely on either an electric or hydraulic system that is manually actuated by the operator.
Should an emergency occur such as a fire it is necessary for the operator to activate a switch to lower the stairs to evacuate the machine or find an alternative way off the machine or equipment. This can be a particular problem if there is a fire in the vicinity of the stair operating switch.
Should the operator forget to retract the stairs and drive the machine off, it is possible to destroy the stairs, causing an expensive repair, downtime and generating safety issues, resulting from damage to an emergency exit.
Another known problem of prior art ladder retraction systems is that, when retracted, the ladder is either held retracted by hydraulic/pneumatic pressure, such as provided by the lifting cylinder, or once retracted a locking pin is used to lock the ladder in place. The locking pin itself can be driven by a small hydraulic/pneumatic cylinder.
Problems with such systems are typically associated with bounce. The ladder can bounce during movement of the vehicle, causing failure of the lifting system. Also, damage caused to the locking pin by ladder bounce can result in failure of the locking pin mechanism.
Where the vehicle has an interlock system, such as a proximity switch, connected to the ladder retraction mechanism or to the locking pin actuating mechanism to prevent movement or starting the vehicle until the interlock is operated, ladder bounce can result in the switch operating thereby stopping or shutting down the vehicle unexpectedly. This results in excess downtime of the vehicle and loss of productivity.
With the aforementioned in mind, it is an object of the present invention to provide a system for retracting a ladder that allows a user to physically halt initial retraction but provides sufficient force to maintain the ladder retracted when retracted.